Summary Historians have devoted little attention to blood transfusion in the nineteenth century. In part, this neglect reflects the presentist assumption that, before Karl Landsteiner's discovery of blood types, this practice would have failed too often to gain currency. Yet, transfusion was in fact the subject of much debate, and was actively practised, primarily by obstetricians on haemorrhaging women. Examining this practice through the conceptual lens of ‘blood clots’, both as noun and as observation, I follow transfusors’ assumptions about the (...) nature of the blood and the problem of its coagulation. Tracing the medicalization of ideas about blood by the century's end, I map this shift onto changing notions about why transfusion was performed, what substance was best employed, and what instrument best fitted that substance's movement into the circulation. In this way, ‘Blood Clots’ reconstructs the discourse surrounding transfusion, extending that discourse to material culture in order to illuminate the rationale that guided transfusion's practice in nineteenth-century Britain. (shrink)